This disclosure relates to segregating fluids for use in wellbores, methods for their manufacture, and methods of use.
Drilling fluids (or “muds”) used in the drilling of subterranean oil and gas wells and other drilling applications are well known. Drilling fluids carry cuttings and other particulates from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling fluid is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that as used herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids.”
A spacer fluid, in contrast, is a liquid used to physically separate one special-purpose liquid from another during a drilling operation. Special-purpose liquids are subject to contamination, for example from the drilling fluid or the wellbore, so a spacer fluid compatible with each is used between the two. For example, a spacer fluid separates drilling fluid from the cement slurry during cementing operations. For cementing, an aqueous spacer or sequence of spacers usually separates the drilling fluid from the cement slurry subsequently pumped downhole. Current industry techniques utilize a design of spacer systems commonly composed of sequences of spacer fluids, for example one or more turbulent spacers, with increasing densities above that of the drilling fluid that is being displaced from the well bore, and less dense than the cement slurry. Such spacer combinations can be referred to as a “spacer train.”
Even with the use of spacer fluids, however, contamination between the various fluids can still occur, particularly at the leading edge of the following fluid. Contamination is particularly problematic where the two fluids have a different viscosity, density, or both. Drilling fluids are especially prone to being incompletely removed. Some spacer fluids are unstable under operating conditions, especially at higher temperatures as they can exist at the bottom of a wellbore. Accordingly, there remains a need in the art for improved methods for preventing contamination between sequentially pumped fluids in a wellbore, especially during cementing operations. In particular, a need remains for improved methods to effectively remove drilling fluids from wellbores.